A digital-to-analog converter (DAC) converts a digital code to an analog signal. Digital-to-analog converters may be used as the interface between the digital world (such as the world of computers) and the analog world. For example, one type of digital code input to a DAC may be a binary digital signal commonly used in computers or other electronics. With a binary digital signal, the digital-to-analog converter may be a binary weighted digital-to-analog converter in which the binary digital signal is translated into the analog signal. The analog signal output from the DAC may take a variety of forms, such as a current, a voltage, or an electric charge.
The circuitry of the DAC may be implemented in several ways, such as by using switches, a network of resistors, capacitors, or current sources. When using current sources for a binary weighted DAC, the currents are generated corresponding to the binary digital input and then summed to produce an analog output.
One characteristic of the digital-to-analog converter is its resolution. The resolution is the number of possible analog output levels the digital-to-analog converter is designed to reproduce. This is usually stated as the number of bits the digital-to-analog converter uses, which for a binary weighted digital-to-analog converter is the base two logarithm of the number of levels. For instance, a 1-bit digital-to-analog converter is designed to reproduce 2 (21) levels while an 8-bit digital-to-analog converter is designed for 256 (28) levels.
To obtain the necessary resolution, prior current source based digital-to-analog converters included a number of current sources that was one less than the resolution of the digital-to-analog converter. For example, a 1-bit digital-to-analog converter included 1 current source to reproduce 2 analog output levels (the first analog output level with the current source on and the second analog output level with the current source off). A 4-bit digital-to-analog converter included 15 current sources to reproduce 16 analog output levels (e.g., the least significant bit including 1 current source, the second least significant including 2 current sources, the third least significant bit including 4 current sources, and the fourth least significant bit (i.e., most significant bit) including 8 current sources). And, an 8-bit digital-to-analog converter included 255 current sources (i.e., 28−1).
Prior current source based digital-to-analog converters were calibrated using a single reference current source. Specifically, the single reference current source was individually compared to each of the current sources in the digital-to-analog converter to adjust each of the current sources to be equal to the reference current source. The prior current source based digital-to-analog converters are one of the fastest conversion methods to generate an analog signal. However, the current source based digital-to-analog converters are typically expensive and very difficult to have resolution greater than 8 bits. This is due to the number of current sources required for current source based digital-to-analog converters. As discussed above, an 8-bit digital-to-analog converter includes 255 current sources and requires comparison of each of the 255 current sources with the reference current source. Accordingly, a need exists for a current source based digital-to-analog converter that is simple in design and operation.